The present invention relates to the field of charged particle beam processing and, in particular, to a method and apparatus for removing material from a semiconductor lithography mask.
Focused ion beam (FIB) systems are widely used in microscopic-scale manufacturing operations because of their ability to image, etch, mill, deposit, and analyze with great precision. Ion columns on FIB systems using gallium liquid metal ion sources (LMIS), for example, can provide five to seven nanometer lateral imaging resolution. Because of their versatility and precision, FIB systems have gained universal acceptance in the integrated circuit (IC) industry as necessary analytical tools for use in process development, lithography mask repair, failure analysis, and defect characterization.
One step in the fabrication of integrated circuits entails the use of lithography. A semiconductor substrate on which circuits are being formed is typically coated with a material, such as a photoresist, that changes solubility when exposed to radiation. A lithography tool, such as a mask or reticle, positioned between the radiation source and the semiconductor substrate casts a shadow to control which areas of the substrate are exposed to the radiation. After the exposure, the photoresist is removed from either the exposed or the unexposed areas, leaving a patterned layer of photoresist on the wafer that protects parts of the wafer during a subsequent etching or diffusion process.
The term mask is used generically herein to refer to any lithography tool, regardless of the type of exposing radiation and regardless of whether the image of the mask is printed once or stepped across the substrate. A mask typically comprises a patterned layer of an absorber material, such as chromium or molybdenum silicide, on a substrate, such as quartz. When the pattern is formed on the mask, it is not unusual for the mask to have defects, such as having absorber material deposited in areas that should be clear, or having absorber missing from areas that should be opaque. The high resolution of FIB systems make them ideal for removing absorber from areas of a mask that should be clear.
The use of a FIB system for removing opaque material from a mask is described, for example, in U.S. Pat. No. 6,042,738 to Casey et al. for xe2x80x9cPattern Film Repair Using a Focused Particle Beam System,xe2x80x9d which is assigned to FEI Company, Hillsboro, Oreg. the assignee of the present invention. FEI Company manufactures focused ion beam systems, such as the Model 800 Series FIB Systems, that have software specifically for facilitating mask repair.
When using a FIB system to remove opaque material from a mask, the mask is positioned on a stage that is maneuvered, for example, using positional information from a previous inspection, so that the defect is within the area scanned by the ion beam. The beam then scans the surface of the area around the defect to produce an image. The defect area is identified and the ion beam current is increased to remove the opaque material.
Ideally, after removal of inadvertently deposited absorber material, the substrate area from which the defect material was removed should have the same characteristics as the area would have had if the area had been manufactured without the defect. Unfortunately, the removal process incidentally and unavoidably changes the properties of the substrate. For example, in a photolithography mask that uses a chromium absorber on a quartz substrate, the quartz at the location where the defect was removed typically loses some of its transparency. This loss is particularly severe for the very short exposing light wavelengths used in modern photolithography tools, with the loss of transparency, typically being between 3 percent and 10 percent.
Overetching, particularly at the edges of the repair, is a primary factor in the loss of transparency. This phenomenon is known as xe2x80x9criverbeddingxe2x80x9d because the etched edges resemble riverbeds when viewed with an electron microscope. One approach to minimizing the riverbed effect to first remove the center portion of the opaque defect, and then apply a lower ion dose to the outside edges to complete the repair. This technique is known as a xe2x80x9cframed repair.xe2x80x9d The 800 Series FIB System from FEI Company can perform a framed repair to the operator""s specification. Even when using a framed repair, riverbeds are typically about 20 nm deep.
Other factors thought to contribute to loss of transparency include the implantation of gallium from the ion beam into the quartz substrate and damage to the quartz itself, due to etching past the chromium or due to crystalline damage cause by impact of the heavy gallium ions. U.S. Pat. No. 6,042,738 describes using a clean-up gas with the focused ion beam to improve the transparency of the substrate after removing an opaque defect.
An object of the invention, therefore, is to remove material from a substrate while minimizing or eliminating changes in characteristics of the substrate.
Applicants have discovered that the effects on a substrate of the focused ion beam removal of material on the substrate is minimized if the ion beam moves initially in a pattern that produces a uneven surface. For example, the beam could move in a pattern in which the sample is milled at a series of non-contiguous points to produce a series of holes, rather than moving in a conventional pattern of overlapping pixels. In a subsequent milling step, the beam moves in a pattern, such as a pattern of overlapping or nearly overlapping pixels, to produce a relatively uniform, planar surface. During the subsequent milling step, an etch enhancing gas is preferably used.
The present invention decreases the size of riverbeds and provides increased substrate transparency at the repaired area compared to prior art repair methods. In some embodiments, the invention reduced the amount of time that the mask needed to be under the ion beam, thereby improving production rates.
The invention is not limited to any particular type of mask or substrate materials, although the benefits of the invention are particularly apparent in milling chromium on a quartz substrate.